One problem related to brake light has been reported for the 2015 GMC Sierra 1500. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2015 GMC Sierra 1500 based on all problems reported for the 2015 Sierra 1500.
The 3rd brake light allowed rain to enter the cab of truck. (gasket failed) headliner, carpet on floor, insulation all ruined. Diagnosed by a "water doctor" at the dealer where truck was purchased. The floor on the passenger front was soaked, headliner is stained. I removed all seats, carpet, insulation and the smell is still there. Called GMC to try to get assistance on repairs. Truck is out of factory warranty. GMC offered $585 leaving a balance of $1300 to be paid by me to replace the headliner. How is it that the 3rd brake light allows rain to enter the cab of the vehicle. Itemized estimate from dealer to replace and put truck back together is $4500-$5000. Money that I don't have. This is a poor design. Did not know/realize that a gasket would fail and allow water to leak inside. The truck is parked in our driveway or on the street. I do not have a garage. I started taking the seats and carpet out the last week of December 2020. Over the next few weeks insulation was removed. The moldy/musty smell remains. GMC was contacted approximately the last of January or the first week of February 2021. I am still in talks with them. Aaron, case manager at GMC with whom I have been dealing. He is trying to figure out someway to give me assistance in getting my truck back together. I don't know how long it has been leaking before I started smelling the musty odor.